[OpenMP] Fix non-determinism in clang copyin codegen

Codegen for OpeMP copyin has non-deterministic IR output due to the unspecified evaluation order in a codegen conditional branch, which makes automatic test generation unreliable. This patch refactors codegen code to avoid this non-determinism.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D101952